1. Field of the Invention
The invention relates to the recovery of the more readily adsorbable component of a gas mixture. More particularly, it relates to the recovery of nitrogen from air using pressure swing adsorption processing.
2. Description of the Prior Art
In numerous chemical processing, refinery, metal production and other industrial applications, high purity nitrogen is desired for purging, blanketing, the providing of metal treating atmospheres, and other purposes. Enriched oxygen gas is also frequently required for various purposes in the same facility. Nitrogen and oxygen can, of course, be obtained by various known techniques for air separation. Pressure swing adsorption (PSA) processing is particularly suited for such air separation in a variety of applications, particularly in relatively small sized operations for which the use of a cryogenic air separation plant may not be economically feasible.
In the PSA process, a feed gas mixture containing a more readily adsorbable component and a less readily adsorbable component are commonly passed to an adsorbent bed capable of selectively adsorbing the more readily adsorbable component at a higher adsorption pressure. The bed is thereafter depressured to a lower desorption pressure for desorption of said more readily adsorbable component and its removal from the bed prior to the introduction of additional quantities of the feed gas mixture to the bed as cyclic adsorption-desorption operations are continued in said bed. As those skilled in the art will readily appreciate, the PSA process is commonly employed in multi-bed systems, with each bed employing the PSA processing sequence on a cyclic basis interrelated to the carrying out of such processing sequence in the other beds in the system.
There have been numerous attempts to enhance the PSA process, particularly to lower capital costs, increase reliability and minimize operating costs, as by achieving relatively low power consumption per unit of product being produced. One desirable goal in the achieving of such overall objectives is to enable the production of relatively high purity coproduct in addition to the desired high purity product. As applied to air separation and other gas separation operations, the Batta patent, U.S. Pat. No. 3,636,679, discloses a PSA cycle as applied to two or more beds wherein each bed is partially repressurized from a lower desorption pressure by simultaneous feed gas-product gas introduction from opposite ends of the bed followed by further repressurization to higher adsorption pressure by feed gas alone, after which the bed is cocurrently depressurized with release of less readily adsorbable component from the discharge end thereof, a portion thereof being recovered as product gas and the remainder being used for pressure equalization and providing purge gas to another bed or beds in the system. The bed is then countercurrently depressurized with release of gas from the feed end of the bed and purged prior to commencing partial repressurization using additional feed gas as cyclic operations are carried out on a continuous basis. The approach of this patent has been successfully employed in air separation operations intended to recover product oxygen as the less readily adsorbable component of air. The Batta process is not applicable, however, to the recovery of the more readily adsorbable component of air, e.g. nitrogen, as the desired high purity product gas.
Various other processes exist, however, in which it is desired to recover the more readily adsorbable component as product gas. Such processes commonly employ a vacuum cycle in which the more readily adsorbable component of the gas mixture is desorbed from the bed at a subatmospheric desorption pressure. Thus, the Tamura patent, U.S. Pat. No. 3,797,201, discloses an air separation process that includes the introduction of air at atmospheric adsorption pressure into an adsorbent bed capable of selectively adsorbing the more readily adsorbable nitrogen component thereof, followed by vacuum desorption to recover said nitrogen as desired product gas. To increase the purity of the product nitrogen, Tamura teaches the carrying out of the initial adsorption step with release of oxygen-rich gas from the discharge end thereof until breakthrough of the adsorption front at said discharge end of the bed and the use of a cocurrent purge at said higher adsorption pressure, using nitrogen for purge, prior to countercurrent vacuum desorption and repressurization. The application of this process tends to be limited by the unavailability of coproduct oxygen at a useable pressure and in an energy efficient manner, although high purity nitrogen product can be obtained thereby. A similar processing cycle is described in the Sircar et al patents, U.S. Pat. Nos. 4,013,429 and 4,264,340, that employs two adsorption trains, each consisting of a pretreatment bed and a main separation bed, together with variable volume surge tanks to accommodate discontinuous flow rates between processing steps.
Vacuum desorption is likewise employed in the process of the Miwa et al patent U.S. Pat. No. 4,070,164, which includes pretreatment for cleaning and drying air and a processing cycle that includes (1) pressurization of a bed to about 4 atm by air feed, (2) cocurrent purge at said elevated pressure with nitrogen to remove an oxygen-rich stream from the discharge end of the bed, (3) countercurrent depressurization to atmospheric pressure with release of nitrogen-rich gas from the feed end of the bed, and (4) vacuum desorption to about 0.3 atm with release of additional nitrogen-rich gas from said feed end of the bed. By the combining of gas released during the two countercurrent depressurization steps, a constant flow of high purity nitrogen is recovered from the system, although the recovery level for the desired nitrogen is quite low using this approach. The same four processing steps were also disclosed in the Armond patent, U.S. Pat. No. 4,129,424, which also provides for the cocurrent purge step to be carried out at a pressure substantially equal to the partial pressure of the nitrogen in the feed gas, thereby significantly reducing the amount of purge gas required to saturate the bed as compared with similar processes in which purging is carried out at a higher pressure. This, in turn, leads to the inclusion of a cocurrent venting step after air feed introduction to reduce the pressure of the bed to that of the purge gas.
Despite such efforts in the art, there remains a need for the development of a PSA process for the production of nitrogen as the selectively adsorbed component of air, wherein the desired product can be recovered at high purity and high recovery levels, together with a valuable production of oxygen-enriched coproduct. Those skilled in the art will also appreciate that there is a similar need and desire in the art for such a process capable of facilitating the recovery of the more readily adsorbable component of a gas mixture, at high purity and recovery levels, together with relatively high recovery of the less readily adsorbable component as coproduct.
It is an object of the invention, therefore, to provide an improved PSA process.
It is another object of the invention to provide a process for the recovery of nitrogen from air by the use of PSA technology.
It is another object of the invention to recover nitrogen at high purity and recovery levels as the more readily adsorbable component of air passed to a PSA system.
It is a further object of the invention to provide a PSA process capable of achieving high purity and high recovery levels of the more readily adsorbable component of a gas mixture, together with relatively high recovery of enriched, less readily adsorbable component gas as a coproduct gas.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.